1. Field of the Invention
This invention relates to a method of cleaving optical fibers, and fibers joined after preparation thereby.
2. Description of the Prior Art
Optical fibers are prepared prior to splicing to another fiber, or joining to a terminating device, by cleaving the fiber to obtain a high-quality endface. In order to obtain low optical losses, the endface of the fiber must be substantially flat and without flaws. One technique for cleaving optical fibers is described in "Optical Fiber End Preparation for Low Loss Splices," by D. Gloge et al, Bell System Technical Journal, Vol. 52, page 1579 (1973). In that technique, a fiber is clamped into position, and a curved form is pressed against the fiber to obtain a curvature. A scoring blade is next passed over the curved fiber. The fiber then typically breaks in such a way as to produce a flat endface; see also U.S. Pat. No. 3,934,773, coassigned with the present invention. In other cleaving methods, a fiber is inserted in a tool, and a scribing wheel is turned to score the fiber substantially around its circumference. A slight tension then results in a break of the fiber. Another technique for cleaving fibers is to manually grasp a section of bare fiber and scribe it across a hard edge, and thereafter pull it straight in order to snap the fiber at the scratch; see "Fibers--Simple Testing Methods Give Users a Feel for Cable Parameters," by R. B. Chesler and F. W. Dabby in Electronics, pages 90-92, Aug. 5, 1976.
In addition to endface quality, one parameter of importance is the angle of the endface to the axis of the fiber. It is desirable that the plane of the endface be normal to the fiber axis, with the "fracture angle" measuring deviation from the normal. It has been found that tortion adversely affects the fracture angle when cleaving a fiber; see "Tortion Effects on Fractured Fiber Ends," by M. J. Saunders, Applied Optics, Vol. 80, page 1480 (1979). The method for measuring or reducing tortion given therein was to hang a portion of the fiber vertically with a weight until it achieved a neutral position. For measuring the effects of tortion, a scored fiber was rotated a certain amount, a tension applied, and the resulting fracture angle measured.
With the advent of single mode optical fibers, the quality of the splice between fibers is especially important. This is because single mode optical fibers can have a very low loss; for example, on the order of 0.3 to 0.5 db/km for radiation having a wavelength of about 1.3 or 1.55 micrometers. It is evident that even very low values of additional loss introduced by splices will significantly limit the spacing between repeaters in such low loss fibers. For example, if a splice is placed on average every kilometer in a fiber that has an inherent loss of 0.4 db/km, and if the splice itself adds an additional 0.2 db loss, the average loss of the spliced fiber will be 0.6 db/km. However, if the splice loss were reduced to 0.1 db, the average loss of the spliced fiber would be 0.5 db/km. For a typical single mode optical fiber system operating in a wavelength range of about 1.3 to 1.55 micrometers, this reduction in loss is estimated to yield an increase in maximum repeater spacing of about 1 to 2 kilometers. Therefore, a very significant economic benefit is realized by reducing the splice loss between low loss fibers. For this reason, special attention must be paid to preparing the ends of optical fibers prior to splicing.